1. Field of the Invention
The present invention relates, in general, to a method of making a multi-ply bottom of clad metal cookware and, more particularly, to a method of making a multi-ply bottom of clad metal cookware by externally integrating an aluminum plate, clad with a stainless steel plate, with the base bottom of the cookware into a single structure through a friction process using an instantaneous super-high pressure, thus almost completely preventing an unexpected removal of the aluminum plate from the bottom, the multi-ply bottom being also designed to allow food in the cookware to be uniformly and appropriately heated without being partially scorched or burnt at a position around the rounded corner of the bottom, thus making food tasty.
2. Description of the Prior Art
In the prior art, most cookware is typically made of a single metal. However, such single metal cookware is problematic in that it may be reduced in heat conductivity when it is designed to have a high degree of corrosion resistance, or it may be reduced in corrosion resistance when it is designed to have a high degree of heat conductivity. The known single metal cookware is thus inconvenient to users.
In an effort to overcome the above-mentioned problems experienced in the single metal cookware, clad metal cookware has been proposed and somewhat effectively used. As well known to those skilled in the art, a clad metal is a sheet material which is formed by integrating two or more different metal plates into a multi-ply metal plate. In the kitchenware art, a stainless steel-clad aluminum plate, or an aluminum plate clad with a stainless steel plate, is typically used as a material of clad metal cookware.
The stainless steel preferably has a high degree of corrosion resistance, but regrettably has a low degree of heat conductivity, thus being problematic in that it causes cookware to be partially heated when the bottom of the cookware is made of the stainless steel. Such a problem of the stainless steel is almost completely overcome when the stainless steel is used with aluminum in a clad metal. For example, a stainless steel plate, or STS304 plate, regrettably has a temperature difference of at least 200.degree. C. between the center and the edge. However, in a clad metal plate consisting of an aluminum plate clad with a stainless steel plate, the temperature difference between the center and the edge is not higher than 25.degree. C. That is, such a clad metal plate has a somewhat uniform temperature distribution. The clad metal plate, consisting of an aluminum plate clad with a stainless steel, has a high induction heating performance when cookware, made of said clad metal plate, is used in an induction heating cooker which has been widely used recently due to a high degree of thermal efficiency and operational stability.
However, known clad metal cookware is problematic in that the aluminum plate, or the main heat transferring plate, is so thin as to render the bottom of the cookware to be partially heated. Therefore, such clad metal cookware has a substantial temperature difference between the bottom, the middle portion of the sidewall, and the top edge portion of the sidewall, thus failing to uniformly heat food. That is, food, contained in such clad metal cookware, is more rapidly heated at the lower portion around the bottom of the cookware than the upper or middle portions. This makes the food undercooked, partially scorched or burnt, and so the food fails to have a good flavor.
In order to solve the problems experienced in such clad metal cookware, a multi-ply bottom brazing clad metal cookware has been proposed and used. An example of known multi-ply bottom brazing clad metal cookware is shown in FIG. 4. As shown in the drawing, the multi-ply bottom brazing clad metal cookware 6 has a multi-ply bottom, or a five-fly bottom 5. The above multi-ply bottom 5 is formed by brazing both an aluminum plate 1 and a stainless steel cladding plate 2 to the external surface of the base bottom of the cookware 6 using a pressing machine, such as a press, or a high-frequency heating machine. In such a case, a bonding material 3 is used in the brazing process. Such a multi-ply bottom 5 maximizes both the heat conductivity and heat conservativity of the cookware 6, thus allowing food in the cookware 6 to be uniformly heated and to have a good flavor. Such cookware 6 also conserves energy while cooking. However, the above multi-ply bottom brazing clad metal cookware 6 has a problem in that it is impossible to integrate the aluminum plate 1 to the rounded bottom of the cookware 6 since a sensitivity heating coil of known brazing machines is designed to only heat a flat surface. Therefore, food is partially scorched or burnt at a portion around the corner of the bottom which is directly affected by gas flames and fails to have a good flavor.